1. Field of the Invention
The present invention relates to a power unit for air conditioning systems principally intended for installation on power boats, such as cabin cruisers or cruising yachts.
2. Discussion of Related Art
A cabin cruiser is a type of power boat that provides accommodation for its crew and passengers inside the structure of the craft. A cabin cruiser usually ranges in size from 25 to 45 feet (7.6 to 13.7 m) in length, with larger pleasure craft usually considered yachts. These craft are generally equipped with a head (toilet), a galley, and at least one berth. Most cabin cruisers usually have a small dining area and some have an aft cabin (a cabin to the rear of the cockpit, with a double bed). Some cabin cruisers are equipped with heating, air conditioning, and power generators. Most also have water heaters and shore power electric systems.
Cruising yachts are by the far the most common yacht in private use, making up most of the 35 to 80 feet (10.7 to 24 m) range. They mainly differ from cabin cruisers because they are more complex in design and equipped with both technological and logistical systems suitable for longer period of stay at sea. So, they may have a large number of cabins below deck, galley, toilet with shower, storage rooms, etc. besides complex navigational and radio-communication systems.
Another category of vessels includes the boat designed for professional use. In many cases, their design is typical and specific for their use and function (e.g.: fishboats, tugs, fire-fighting boats, etc.), but in many cases they are derived from cruisers or yachts, with just some adaptation. This last category includes most of patrol boats for Coast Guard, Police, Customs & Excise duties. On the other hand can be said that marine designers and shipyards, to reduce costs, design boats that can easily and economically adapted to very different purposes in both leisure and professional fields.
The boats of the categories described above can adopt a very variable range of power plants, both in type and power installed. It is possible find small cruisers with gasoline outboard motors (tens of kW of installed power) and yachts with diesel engines and shaftlines (thousands of kW of installed power). It can be said that gasoline outboard motor propulsion, also in twin or triple configuration, with engines each up to 300-350 hp can be found in the top range of leisure boats. The very same boats are often made available by the shipyards also in inboard gasoline or diesel engines configuration. Gasoline fuel engines are more common in some areas of the world (such as the USA and the Arabic Gulf) where gasoline fuel cost is lower and for power up to about 500 hp each, while diesel is largely spread all over the world, for any power range and type of use. Diesel engines characteristics make them suitable to be coupled with any type of available marine propulsion system, ranging from immersed propellers, to stern drives, water jets, pods and surface drives. Gasoline inboard engines are normally coupled to stern drives or surface drives.
The large number of boats, of any type, also of medium-small size, fitted with one or more cabins or, generally, one or more closed spaces, makes the air conditioning system considered desirable and essential by almost all boat owners, at least in warm and hot areas of the world, even as an aftermarket installed accessory.
Depending on the type of boat, systems having different concepts can be used. However, even for the smallest boats (i.e. with only one room to be warmed/cooled) the energy source remains the electrical current, which is generated by a generator set or by the shore power line if the boat is in port. This activates an electric motor, which drives the compressor of the conditioning system.
In the case of boats of larger dimensions, with more than one room (or cabin) to be warmed/cooled, centralized systems are installed. These can be of two types: with direct distribution of a thermal exchange fluid to the evaporators or with circulation of treated water, i.e. previously warmed/cooled by a conditioning device through a heat exchanger, sent to one or more fan coils.
In any case, all air conditioning systems available on the market require the presence of a generator set capable of supplying the necessary power for an electrically operated compressor.
In fact, in air conditioning systems installed on boats, the compressor is a device operated by an electric motor, which, owing to the fact that it can also be powered directly by the shore power network, requires an alternating current power supply. The choice of using alternating current power supply, instead of continuous current taken from batteries of on board electrical system (that is typically 12V or 24V) is easily explicated. The higher the voltage can be supplied to the electrical machine, the higher the power that can be obtained with the same current intensity or, on the other side, lower current is required to obtain the same power output. It is important to remind, also, that the higher the current, the larger the section of wiring required to reduce electrical resistance and dissipation. This means also higher weight and cost of the electrical wiring.
The rated installed power of the compressor (conventionally indicated with the unit of measure BTU/h) depends on the size of the system, i.e. on its characteristic parameters, namely the volume to be warmed/cooled, the difference in temperature to be imparted, the thermal exchange fluid used (i.e. its specific heat) and the temperature of the water in the operational area (that acts as coolant for the open circuit in case of cooling). The higher the installed power of the compressor, the higher the electrical power required and the larger the dimensions, cost and weight of the power generator set.
For the reasons stated above, the electrical power required by the compressor motor, as already mentioned, is normally supplied by the electric generator of a generator set installed for this purpose, which must also supply the accessories of the system, such as the water circulation pumps and the fan coil fans. Alternatively, when the boat is moored in an appropriately equipped port, the power supply to the compressor motor and to the aforesaid accessories can be provided by the shore power network.
Nonetheless, due to the high cost and considerable weight added, the procedure depicted above can be rarely performed. A generator set capable of supplying utilities with alternating current is rarely present on small and medium sized pleasure boats, where the on board batteries charged by the alternators of the main motors are generally sufficient to supply current to the on-board services, systems and electrical appliances and this absence makes it not practicable the installation of an air-conditioning system.